Toxins are extremely potent cell-killing agents that are responsible for many human diseases. Because of their high activity, these agents have been attached to monoclonal antibodies in order to form cytotoxic agents (immunotoxins) which specifically bind to target cells. These immunotoxins are, therefore, most useful in cancer therapy.
Pseudomonas exotoxin A (PE) is an extremely active monomeric protein (molecular weight 66Kd), secreted by Pseudomonas aeruqinosa, which inhibits protein synthesis in eukaryotic cells through the inactivation of elongation factor 2 (EF-2) by catalyzing its ADP-ribosylation (catalyzing the transfer of the ADP ribosyl moiety of oxidized NAD onto EF-2).
The intoxication process is believed to proceed by the following steps: First, PE binds to cells through a specific receptor on the cell surface. Next, the PE-receptor complex is internalized into the cell. Finally, PE is transferred to the cytosol where it enzymatically inhibits protein synthesis. The transfer process is believed to occur from an acidic compartment, since cellular intoxication is prevented by weak bases such as NH.sub.4 +, which raises the pH in acidic vesicles. Upon exposure to acidic conditions, the hydrophobic domain of PE enters into the membrane, resulting in the formation of a channel through which the enzymatic domain, in extended form, pass into the cytosol.
PE-containing immunotoxins are constructed by first reacting native PE with iminothiolane. This reaction serves both to introduce two new sulfhydryl groups used for coupling an antibody to the toxin, and to inactivate the binding of PE to its own receptor. This approach relies on the chemical inactivation of PE-binding sites in order to minimize undesirable side effects due to the binding of PE to cells with PE receptors. While this approach has been reasonably successful in producing a specific cell killing reagent in tissue culture and in tumor-bearing mice, it has not been possible to administer more than 2 ug of PE immunotoxin to a 20 gram mouse or 1 mg to a 3 kilogram monkey or 4 mg to an adult human, due to the toxic side effects of the immunotoxin. It is therefore desirable to be able to administer larger amounts of immunotoxins to achieve greater killing of tumor cells. The present invention fulfills this desire by providing an immunotoxin with high potency and low toxicity. Furthermore, to overcome the above-noted reliance on chemical inactivation of the PE binding sites, the present invention incorporates recombinant DNA techniques to clone the complete toxin gene (or segments of it) in order to express at high levels the full length toxin molecule (or portions of it) containing different functional domains, including one which lacks the cell binding domain. See Example 1 for a comparative examination of these clones.
The three-dimensional structure of PE has been determined by x-ray crystallography. As shown in FIG. 2, the PE molecule contains three structurally distinct domains: Domain I contains amino acid residues 1 to 252 (Domain Ia), and 365 to 404 (Domain Ib); domain II contains amino acid residues 253 to 364; and domain III contains amino acid residues 405 to 613.
Plasmids have been constructed which express various portions of the PE molecule, providing the ability to correlate different structural domains with various functional activities and to determine (1) which portions of the molecule are responsible for cell recognition (binding), (structural domain I, amino acid 1-252); (2) which portion is required for enzymatic activity (ADP ribosylating activity, domain III plus a portion of domain Ib) (amino acid 385-613); (3) which portion is responsible for translocation across cell membrane (domain II). The evidence that structural domain Ia is involved in cell recognition is that proteins produced by plasmids without domain Ia but containing domain II, Ib and III, are not cytotoxic by themselves and do not show competitive inhibition of the cytotoxicity of intact toxins, whereas a plasmid encoding domain Ia, but missing other domains, blocked PE cytotoxicity of sensitive cells. PE from plasmids with a deletion of the first half of structural domain II exhibit both PE blocking activity and ADP-ribosylation activity, but these molecules lost all cell killing activity. Plasmids which encode only structural domain III produce large amounts of the protein, but lack detectable enzymatic activity (ADP-ribosylation). However, plasmids which encode all of structural domain III plus adjacent amino acids from structural domain Ib express large amounts of the protein and their ADP-ribosylation activity is high.
Based on the three dimensional structure of PE, plasmids have been constructed which express different portions of PE. The protein pattern of the cells expressing the different constructions was analyzed by SDS gel electrophoresis and the ADP ribosylating activity of the recombinant toxins was measured. Of these plasmids (described in Example 1) plasmid pJH8, containing amino acids 253 to 613 (Domains Ib, II, and III), and plasmid pJH17, containing amino acids 385 to 613 (Domain III, plus 20 adjacent amino acids from Domain Ib), are able to encode large amounts of modified PE exhibiting low toxicity to human cells, but remaining enzymatically very active.
Taken together, the plasmid patterns indicate that structural domain Ia is a receptor binding domain, that the first half of structural domain II is required for translocation of the toxin from a host cell's endocytic vesicle to the cytosol, and that structural domain III alone is not sufficient to express full ADP-ribosylation activity.
When administered to animals, PE characteristically produces death due to liver failure. Immunotoxins made with PE also attack the liver and, when given in large amounts, produce death due to liver toxicity. The experiments shown in Table III indicate that Domain Ia is responsible for cell binding and indicate that a PE molecule in which Domain Ia is deleted is less toxic to mice than native PE. The data shown in Example 4 support this conclusion, indicating that modified PE is at least 200-fold less toxic to mice than native PE. Example 5 shows that when the protein made by pJH8 is purified and coupled to an antibody to the human transferring receptor, an immunotoxin is created that is approximately as active as an immunotoxin containing native PE but is 100-fold less toxic on nontarget cells (mouse cells).
One aspect of the present invention, therefore, is that PE molecules with a deletion of Domain Ia are effective immunotoxins with diminished side effects including diminished liver toxicity.